Plasma extraction guns and applications therefor



April 23, 1968 P. CHORNEY ETAL 3,379,910

PLASMA EXTRACTION GUNS AND APPLICATIONS THEREFOR Filed July 9, 1965 4Sheets-Sheet 1 INVENTORS. 5 P601, 6/weA/67 BY JEN/V V 74 N 2/17 E ha MFiled July 9, 1965 P. CHORNEY ETAL 3,379,910

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A ril 23, 1968 P. CHORNEY ETAL PLASMA EXTRACTION GUNS AND APPLICATIONSTHEREFOR 4 Sheets-Sheet 5 Filed July 9, 1965 JV 254 780 Ix 7552370 Woe/cP/c as M NM Z a j 5 I a M f a MUM L W I N VENTORfi- WWW Pv L 5 M 7! m L7 f 0 7 3%? W April 23, 1968 P. CHORNEY ETAL 3,379,910

PLASMA EXTRACTION GUNS AND APPLICATIONS THEREFOR Filed July 9, 1965 4Sheets-Sheet L INVENTORj. Paw, awe/v5 Ja/m/ mum United States Patent3,379,910 PLASMA EXTRACTEUN GUNS AND APPLICATIONS THEREFOR Paul Chorney,Norwood, and John Valun, Framingham, Mass., assignors, by mesneassignments, to the United ISqtates of America as represented by theSecretary of the avy Filed July 9, 1965, Ser. No. 470,944 5 Ciaims. (Cl.31363) ABSTRACT 0F THE DISCLOSURE A plasma is created and a highperveance beam is extracted from the plasma reliably and stably overlong periods of time. Three stacked panallel flat electrodes supportedin an atmosphere of 1-100 microns and normal to a unidirectionalconstant magnetic field generate the plasma. The two outer electrodesare connected in common and have one or more in-line perforation pairs.The intermediate electrode is at about fifteen volts positive relativeto the other two electrodes. One of the other two electrodes is ofnickel cermat thermionic emissive material and has a skirt directed awayfrom the intermediate electrode and supports a heater therein. Theintermediate electrode has a hole slightly larger than the outidedimension of the emitter electrode. At least one extraction anode issupported parallel with but beyond the repeller electrode and has aperforation in-line with perforations in the commonly-connectedelectrodes and at a high positive potential relative to the three plasmaforming electrodes. A collector electrode is supported beyond theextraction anode or beyond the emitter electrode in the line ofparticles that issue from the perforation in the adjacent electrode.

This invention relates to improvements in technique for extracting abeam of charged particles from a plasma and to applications therefor.

The Philips Ion Gage or Penning Ion Gage (P.I.G.) type of device hasbeen used for beam extraction from plasma. The subject is treated atlength in a report entitled Extraction and Modulation of Electron Beamfrom Philips Ion Gage by R. E. Lundgren, Scientific Report No. 9 ofElectronics Research Laboratory of University of California, Berkeley,Calif, 1960, Astia Number AD 247,432. The basic components of the P.I.G.include a cylindrical electrode whose axis is normal to two fiatelectrodes, one beyond each end of the cylindrical electrode. A ring orperforated plate are alternatives to the cylindrical electrode. Theelectrodes are supported in a gaseous atmosphere at a pressure below l()torr (1 micron). A undirectional constant magnetic field is directedaxially of the cylindrical electrode and a positive potential is appliedthereto relative to the two flat electrodes sufficient to generate aplasma between the flat electrodes. The required voltage depends uponthe fiat electrode material, the gas pressure, and the particular gas. Abeam of electrons or positive ions has been extracted from this type ofdevice by perforating one of the fiat electrodes and disposing anotherelectrode beyond the perforated electrode to apply a suitableaccelerating potential.

Ion bombardment limits the capability of these devices to provide highperveance beams, limits their useful life to an impractically shortperiod, and limits their utility to extremely low pressure environment.

Perveance (K) of an electron gun is defined as where I is in amperes andV is in volts. Perveance is considered high when it is greater than 1l0" amps/ volts The previously known P.I.G. type devices are not usefulfor high perveance beams.

In P.I.G. type devices electrons in the gaseous atmosphere acceleratetoward the cylindrical electrode or anode from the end electrodes orcathodes but are confined to a spiral path by the axial magnetic fieldand on approaching either end electrode or cathode are repelled. Theelectrons oscillate axially through the anode until they collide with agas atom or molecule and ionize it. The newly created slow electronoscillates in the same manner as the ionizing electron and the positiveion is accelerated directly into the nearer cathode. In this manner, aplasma is created which reaches an equilibrium state determined by therate at which electrons reach the anode. Electrons reach the anode as aresult of multiple collisions with gas particles.

The Voltage required for the P.I.G. is determined by the work functionand temperature of the cathode, the type of gas, the pressure, themagnetic field, and the plasma density. This voltage ranges from severalhundred to several thousand volts depending on the choice of panameters.Unless the gas pressure and plasma density are very low, the cathodesundergo severe ion bombardment; therefore these devices have beenlimited to gaseous pressures :below a micron and for short periods oftime until the electrodes and supporting insulators have been renderedunsatisfactory for proper operation by ion bombardment and sputtering.

An object of this invention is to provide a plasma extraction gun forsupplying a high perveance beam reliably over long periods of time.

A further object is to provide a beam plasma amplifier with a reliableelectron beam source.

A further object is to provide an electron beam source of high perveancefor electron beam welding and various scientific studies.

A further object is to provide an ion beam source for target bombardmentand sputtering, for scientific studies, and for ion engine propulsion.

A further object is to provide a more reliable, more durable, morestable, and generally superior beam extraction gun.

Other objects and advantages will appear from the following descriptionof an example of the invention, and the novel features will beparticularly point out in the appended claims.

PEG. 1 shows an embodiment of an electron beam extraction gun, partly insection, partly in elevation, in simplified form, omitting parts notessential to illustrate the principles of this invention;

FIG. 2 is a semi-log plot of retarding potential characteristic of theembodiment in FIG. 1 wherein the negative retarding potential is appliedto the first anode.

FIG. 3 shows a fragment of the embodiment of FIG. 1 modified for ionbeam extraction,

FIG. 4 illustrates in block form a beam plasma amplifier embodying thisinvention,

FIG. 5 illustrates in block form an electron beam welding arrangementembodying this invention,

FIG. 6 illustrates in block form an ion beam sputtering or bombardingarrangement embodying this invention, and

FIG. 7 illustrates another structural arrangement embodying the sameprinciples as the structure of FIG. 1.

This invention concerns the modification of a PlG. type of dischargedevice. In this invention one of the cathodes is a thermionic emitterand has an ion extraction hole in line with an electron extraction holein the other cathode and is formed of nickel cermet, for example, whichis exceptionally resistant to ion bombardment and an excellent emitter.The discharge is operated at a very low voltage, e.g., 15 voltsminimizing ion bombardment. The gas pressure is substantially higherthan in a P.I.G. making possible a high plasma density and a highperveance beam. The invention contemplates a cathode that has aplurality of extraction holes, the same number in both cathodes andwherein corresponding holes are colinear.

The embodiment shown in FIG. 1 includes an anode 10, and cathodes 12 and14 corresponding to the electrodes of a PIG. The electrodes have radialsymmetry and are affixed to three parallel insulating support rods 16,only two of which are shown. The anode is a washer shaped member. Thecathode 12 is a cylindrical member whose outside diameter is smallerthan the inside diameter of the anode and preferably is of nickel cermetdescribed in US. patent application Ser. No. 375,986, filed June 17,1964, now Pat. No. 3,303,378, for Cathode by Paul Chorney and John Valunand assigned to the US. Government. The end of the cathode 12 directedtoward anode 10 is flat, normal to the axis of the electrodes, and isformed with an axial perforation 22. A heater coil 24 is supported inthe cathode. The cathode i2 is mounted in a cathode support 26 andoperates as a thermionic emitter. The other cathode 14 is a repeller andis formed with an axial perforation 28 which has a tapered openingnarrowing down to a diameter at the smaller end equal to the diameter ofthe perforation 22 in the cathode. The anode 10 is intermediate the endface of cathode 12 and the repeller 14. Two electron beam extractionanodes 30 and 32 are supported beyond the repeller 14 and have axialperforations 34 and 36 respectively of essentially the same diameter asthe perforation in the cathode. An ion collector 38 is supported beyondthe cathode 12. A collector or target for the electron beam is not shownin FIG. 1. To inhibit diffusion of the plasma discharge outside of thebounds of electrodes 10, 12, and 14 and causing supurious discharges, aglass sleeve 40 is bonded at one end to the repeller and extendscoaxially therefrom toward and beyond the emitter cathode 12 and servesas a diffusion shield. The gun is confined in a gastight glass envelope42 in which the selected gas atmosphere at the selected pressure isestablished by conventional techniques. As an alternative to thediffusion shield, the glass envelope may be necked down around andagainst the repeller.

Molybdenum is a satisfactory material for the electrodes other than theemitter cathode.

One set of dimensions for the gun in the illustrated embodiment is asfollows. The diameter of cathode 12 is 0.250 inch and the diameter ofthe perforation 22 is 0.040 inch. The anode 10 has an inside diameter of0.300 inch and the perforation 28 is the repeller 14 narrows downthrough a 120 degree taper to 0.040 inch, the same diameter as theperforation in the cathode. The first anode 30 is spaced 0.060 inch fromthe 0.040 inch end of the perforation in the repeller and is operable toextract the electron beam current from the discharge. The second anode32 post accelerates the extracted bean and determines the beam voltage.

The disclosed embodiment may be operated with a cathode temperature onthe order of 800 C., an anode voltage which is a small fraction of 100volts, e.g., as low as volts, and a second extraction anode voltage onthe order of l kv. If the first extraction anode is now operated at apositive potential on the order of 200 volts, a very high perveanceelectron beam is extracted. A beam of micropervs was obtained with theembodiment described. To lower the perveance a lower or negativepotential is applied to the first extraction anode; a negative potentialturns back a percentage of the electrons tending to leave the dischargethereby lowering the perveance of the beam. If a semi logarithmic plotis made of the retarding potential characteristic, i.e., beam current inmilliamps plotted logarithmically as the ordinate and retardingpotential on the first extraction anode as the abscissa, the plotappears very nearly linear on these coordinates, as illustrated in FIG.2, and has the same qualitative characteristic exhibited bythermionically emitting cathodes. The retarding potential characteristicof common cathodes is governed by the law where: I is the cathodecurrent with zero potential applied to the anode,

e is the charge of an electron,

V is the retarding potential,

k is Boltzmanns constant,

T is the absolute temperature of the cathode.

Assuming that the virtual cathode of the plasma extraction gun isgoverned by the same law, an effective temperature of the virtualcathode may be inferred from the slope of the retarding potentialcharacteristic. The resulting effective temperature with the structuredescribed was found to be approximately 100,000 degrees Kelvin. I

Although there are high densities of ions formed in the plasmadischarge, the damage to the emitter cathode and the repeller due to ionbombardment is minor because of the low discharge voltage (about 15volts) in comparison with beam voltages (about 1 kv. and greater) andbecause of the use of nickel cermet for the emitter cathode. High energyions formed in the electron beam which can bombard only a small portionof the cathode surface in line with the beam cause no damage because ofthe perforation in the cathode, directly opposite the hole in therepeller, that serves as a drain for the high energy ions coming fromthe electron beam region. High emission densities were obtainedcontinuously and high perveances were obtained with extraction guns ofthe type described.

In the illustrated embodiment, there is only one perforation in theemitter cathode and in the repeller. For two or more beams the anode andthe repeller are formed with a plurality of colinear perforationssubstantially as the one set shown in FIG. 1.

In FIG. 3, there is shown a fragment of the beam extraction gun of FIG.1 modified to provide an ion beam. The ion collector 38 of FIG. 1 isperforated to provide an ion beam extractor 38a. A negative acceleratingpotential is applied to beam extractor 38a. The repeller 14 need not beperforated and the anodes 30 and 32 are omitted in an ion beamextraction gun. Also, an ion beam may be derived from the embodimentshown in FIG. 1 when large negative potentials are applied to the firstextraction anode 30. The ion beam is post accelerated by also applying alarge negative potential to the second extraction anode 32.

In a beam plasma amplifier a beam of electrons modulated with amicrowave signal is directed through a plasma for amplification of themicrowave signal. The plasma density required for amplification is high,requiring gas pressure on the order of 1 to microns. When a conventionalgun is operated in a gaseous environment at these pressures, the gun isshort lived. The gun undergoes severe ion bombardment Which sputters andpoisons the emitting surfaces. In FIG. 4, there is shown a beam plasmaamplifier embodying this invention. An electron beam extraction gun 50,a plasma interaction region 52 and an electron beam collector 54 aresupported in line in a gas-tight envelope 56 in which the selected gasat the selected pressure is established by conventional techniquesthrough a valve 58 which is closed when the desired environment isestablished in the envelope.

The beam extraction gun 50 takes full advantage of the gas atmosphererequired for the plasma amplifier. The constricted end of the taperedhole in the repeller is, in effect, the virtual cathode for the electronbeam. Compared with conventional microwave tubes, the gas pressure in abeam power amplifier is very high, on the order of l to 100 microns. Therelatively high pressure is necessary for the production of the plasmadensity required for amplification. Of course it is obvious that theelectron beam derived from the extraction gun can be used forinteractions with media other than plasmas.

A high perveance electron beam is useful for specialized weldingapplication. In FIG. 5, there is shown an electron beam extraction gunas in FIG. 1 wherein the envelope 58 has a separable part 66 to permitinsertion and removal of a workpiece 62 which functions as the beamcollector. The gun may be single beam or multibeam for this application.

In FIG. 6, there is shown an arrangement similar to that in FIG. 5 foran ion beam gun wherein a workpiece 62a is bombarded, sputtered, etc. bythe ion beam.

In the electron beam extraction gun embodiment illustrated in FIG. 7,the electrodes and spacers form the container. This embodiment includesa plurality of circular electrodes corresponding to those in FIG. 1 inspaced coaxial relationship. The electrodes include an ion collector 70,a thermionic emitter cathode 72 welded into a conductive support 74, adischarge anode '76, a discharge repeller 78, a first beam extractionanode 80 and a second or post accelerator anode 82. Target is not shown.

The embodiment in FIG. 7 is more compact, simpler to operate, isgenerally superior for scientific studies of controlled variation ofparameters on beam extraction. By perforating the plate 70 and applyinga negative potential thereto, a structure as in FIG. 3 is operable as anion beam source or an ion propulsion engine: alternatively, this mode ofoperation can be effected by applying a high negative potential to theanode 80.

It will be understood that various changes in the details, materials,and arrangements of parts (and steps), which have been herein describedand illustrated in order to explain the nature of the invention, may bemade by those skilled in the art within the principle and scope of theinvention as expressed in the appended claims.

We claim:

1. An electron gun for use in a low pressure gaseous atmosphere andaligned with a unidirectional constant magnetic field comprising:

a flat electrode,

a thermionic emitter cathode having a heater therein and having a flatface directed toward and parallel to the flat electrode,

means connecting said flat electrode and said thermionic emitter incommon,

an electrode supported between the fiat electrode and the thermionicemitter cathode and having a perforation therethrough in line with andlarger than the flat face of the thermionic emitter cathode,

said thermionic emitter cathode and said fiat electrode having colinearperforations therethrough, and

an extraction anode supported beyond the fiat electrode and perforatedcolinearly with perforations in the emitter cathode and flat electrode.

2. In an electron gun as defined in claim 1, wherein said thermionicemitter cathode is of nickel cermet.

3. An electron gun for use in a low pressure gaseous atmosphere andaligned with a unidirectional magnetic field comprising:

a flat electrode,

a thermionic emitter cathode having a heater therein and having a flatface directed toward and parallel to the fiat electrode,

means connecting said fiat electrode and said thermionic emitter incommon,

an electrode supported intermediate the flat electrode and thethermionic emitter cathode and having a perforation therethrough in linewith and larger than the fiat face of the thermionic emitter cathode,

said thermionic emitter cathode and said flat electrode having colinearperforations,

an electrode supported on the side of said cathode opposite theintermediate electrode to intercept ions issuing from the perforatedcathode,

electrode means for extracting an electron beam from the dischargesupported on the side of the flat electrode opposite the intermediatefiat electrode.

4. An electron gun as define in claim 3, further comprising:

direct current means establishing a positive potential on theintermediate electrode that is about fifteen volts relative to both thethermionic emitter cathode and the fiat electrode.

5. An electron gun for use in a low pressure gaseous atmosphere andaligned with a unidirectional constant magnetic field comprising:

a cup-shaped electrode of thermionic emitter material supporting aheater therein and having a flat end with at least one perforationtherethrough,

a fiat repeller electrode supported in parallel with and spaced from theflat end of the cup-shaped electrode and having a perforation in linewith each perforation in the cup-shaped electrode,

means connecting in common the cup-shaped electrode and the repellerelectrode,

a flat anode supported between and parallel to the flat end of thecup-shaped electrode and the repeller electrode and formed with a holein line with and slightly larger than the outside dimension of the fiatend of the cup-shaped emitter,

means connected to the three electrodes to render the anodeapproximately fifteen volts positive relative to the cup-shapedelectrode and repeller electrode,

a first extraction anode supported beyond the repeller electrode andhaving a perforation in line with perforations in the repeller electrodeand cup-shaped electrode,

a second extraction anode supported between the repeller electrode andthe first extraction anode for adjusting beam perveance,

means connected to the extraction anodes, flat anode, repellerelectrode, and cup-shaped electrode to render the first extraction anodeapproximately onethousand volts positive and the second extraction anodeapproximately two hundred volts positive relative to the otherelectrodes, and

a collector electrode free of perforations supported beyond thecup-shaped electrode.

References Cited UNITED STATES PATENTS 2,137,198 11/1938 Smith 313-1612,141,654 12/1938 Kott 3l3-250 X 12,212,643 8/1940 Koros et al. 313-250X 2,376,439 5/1945 Machlett et al. 313346 2,486,134 10/1949 Elder 3131953,303,378 2/1967 Chorney et al. 313346 FOREIGN PATENTS 205,632 11/1955Australia. 766,171 5/ 1954 Germany.

JOHN W. H'UCKERT, Primary Examiner.

A. 1. JAMES, Assistant Examiner.

